1. Field of the Invention
The present invention relates to a light guide optical system, a document illuminating device using the same, and an image reading apparatus using the same. More specifically, the present invention relates to an image reading apparatus that illuminates a document surface and performs image reading in a line-sequential manner, for example, an image scanner, a copying machine, or a facsimile.
2. Description of the Related Art
In general, a document illuminating device used by an image reading apparatus that illuminates a document surface and performs image reading in a line-sequential manner, for example, an image scanner, a copying machine, or a facsimile illuminates a document surface using a tubular (linear) light source such as a fluorescent lamp.
FIG. 5 is a sectional view of a main portion of a typical image reading apparatus. Illuminating light emitted from a tubular light source 005 illuminates a document placed on a document table glass 001. Light beams reflected on the document surface and having image information are reflected on a plurality of reflecting mirrors 006 and are then caused by an imaging lens 007 to form an image on one-dimensional photoelectric conversion elements (CCDs) 008 arranged in the main scanning direction.
A carriage 004 is moved by a subscanning-direction drive unit 009 and reads a two-dimensional image of the document in a line-sequential manner. In general, a cold cathode fluorescent tube or a xenon tube is used as the tubular light source 005. Xenon tubes are stable in light quantity and are therefore widely used by apparatuses for business use but are relatively expensive. Therefore, apparatuses for home use, whose unit prices are low, use cold cathode fluorescent lamps, which are inexpensive.
However, cold cathode fluorescent lamps are unstable in light quantity. Therefore, development of a linear illuminating device that is inexpensive and stable in light quantity is required.
As a result of recent technological development, luminous efficiency of light emitting diodes (LEDs) has been improved, and a technology to make a linear illuminating device with LEDs has been developed.
When point light sources such as LEDs (light emitting diodes) are used as light sources, a plurality of point light sources are arranged in the main scanning direction.
When a reading surface such as a document surface is directly irradiated with light beams emitted from each of a plurality of point light sources, the illuminance is not uniform in the main scanning direction, and the density of the read image is not uniform.
To restrain the non-uniformity of illuminance in an image reading apparatus using a plurality of point light sources, various technologies have been developed (for example, Japanese Patent Laid-Open Nos. 2005-156600 and 7 -203128).
For example, Japanese Patent Laid-Open No. 2005 -156600 discloses to eliminate the non-uniformity of illuminance by providing a light diffusing portion that randomly diffuse light between light sources and an object (document).
However, the technology described in Japanese Patent Laid-Open No. 2005- 156600 diffuses light beams randomly and therefore even toward the outside of a predetermined region to be illuminated, and therefore suffers from low light use efficiency of light beams emitted from the light sources.
In addition, a flare phenomenon, in which light beams reflected on a region other than the region to be illuminated fall on the region to be illuminated, occurs and can degrade the quality of the image reading apparatus.
Document illuminating devices used in image reading apparatuses need to read a document with uniform quality, and therefore it is desirable that the angular characteristics of light beams incident on the document surface be uniform.
Therefore, it is necessary to illuminate individual positions in the main scanning direction on the document surface from many directions, and to increase the illuminating angle so that a plurality of light sources illuminate a broader range.
However, increasing the illuminating angle of diffuse light beams diffused by a light diffusing portion causes an absorption reflection phenomenon in the light diffusing portion, and further lowers the light use efficiency.